1. Field of the Invention
The present invention relates to an Fe-Ni alloy sheet for a shadow mask used for a color cathode-ray tube and a method for manufacturing same.
2. Related Art Statement
Along with the recent tendency toward a higher-grade color television set, a 36 wt. % Ni-Fe alloy known as the invar alloy is attracting the general attention as an alloy for a shadow mask capable of coping with problems such as a color-phase shift. The invar alloy has a far smaller thermal expansion coefficient as compared with a low-carbon steel conventionally applied as a material for a shadow mask.
By making a shadow mask from the invar alloy, therefore, even heating of the shadow mask by an electron beam would hardly cause such problems as a color-phase shift resulting from thermal expansion of the shadow mask.
However, the invar alloy sheet for a shadow mask, i.e., the original sheet prior to forming by etching passage holes for the electron beam (hereinafter simply referred to as the "holes") has the following problems:
(1) Poor etching pierceability: Because of a high nickel content in the invar alloy, the invar alloy sheet for a shadow mask has, during the etching-piercing, a poor adhesivity of a resist film onto the surface of the invar alloy sheet, and a lower corrosion by an etching solution as compared with a low-carbon steel sheet for a shadow mask. This tends to cause irregularities in the diameter and the shape of the holes pierced by etching, thus leading to a seriously decreased grade of the color cathode-ray tube.
(2) Seizure easily occurring in the annealing of a flat mask: The invar alloy sheet for a shadow mask as pierced by etching, i.e., the flat mask, is press-formed into a curved surface to match with the shape of the cathode-ray tube. To improve press-formability, the flat mask is annealed prior to the press-forming. It is the usual practice, at cathode-ray tube manufacturers, to anneal several tens to several hundreds of flat masks placed one on the top of the other at a temperature of from 810.degree. to 1,100.degree. C., which is considerably higher than the annealing temperature of a flat mask made of a low-carbon steel, with a view to improving productivity.
Since the invar alloy has a high nickel content, it has a higher strength than a low-carbon steel. A flat mask made of the invar alloy must therefore be annealed at a higher temperature than in a flat mask made of a low-carbon steel. As a result, seizure tends to occur in a flat mask made of the invar alloy during the annealing thereof.
For the purpose of solving the problem (1) as described above, the following prior arts are known:
(a) Japanese Patent Provisional Publication No. 61-39,344 discloses limitation of the center-line mean roughness (Ra) of an alloy sheet for a shadow mask within a range of from 0.1 to 0.4 .mu.m (hereinafter referred to as the "prior art 1").
(b) Japanese Patent Provisional Publication No. 62-243,780 discloses limitation of the center-line mean roughness (Ra) of an alloy sheet for a shadow mask within a range of from 0.2 to 0.7 .mu.m, limitation of the average peak interval of the sectional curve representing the surface roughness within a standard length to up to 100 .mu.m, and limitation of the crystal grain size to at least 8.0 as expressed by the grain size number (hereinafter referred to as the "prior art 2").
(c) Japanese Patent Provisional Publication No. 62-243,781 discloses, in addition to the requirements disclosed in the above-mentioned prior art 2, limitation of Re, i.e., the ratio .alpha..sub.1 /.alpha..sub.2 of the light-passage hole diameter (.alpha..sub.1) to the etching hole diameter (.alpha..sub.2) to at least 0.9 (hereinafter referred to as the "prior art 3").
(d) Japanese Patent Provisional Publication No. 62-243,782 discloses that the crystal texture of an alloy sheet for a shadow mask is accumulated through a strong cold rolling and a recrystallization annealing, the crystal grain size is limited to at least 8.0 as expressed by the grain size number, and the surface roughness described in the above-mentioned prior art 2 is imparted to the surface of the alloy sheet for a shadow mask by means of the cold rolling with the use of a pair of dull rolls under the reduction ratio within a range of from 3 to 15% (hereinafter referred to as the "prior art 4").
In order to solve the problem (2) as presented above, on the other hand, the following prior art is known:
(e) Japanese Patent Provisional Publication No. 62-238,003 discloses limitation of the center-line mean roughness (Ra) of an alloy sheet for a shadow mask within a range of from 0.2 to 2.0 .mu.m, and limitation of the value of the deviation index (Rsk) in the height direction of the roughness curve to at least 0 (hereinafter referred to as the "prior art 5").
However, the above-mentioned prior arts 1 to 4 have the problem in that while it is possible to improve etching pierceability of the alloy sheet to some extent, it is impossible to prevent seizure of the flat masks occurring during the annealing thereof.
The above-mentioned prior art 5 has, on the other hand, a problem in that, while it is possible to prevent seizure of the flat mask made of a low-carbon steel during the annealing thereof to some extent, it is impossible to prevent seizure during annealing of the flat mask made of the invar alloy which requires a higher annealing temperature than the low-carbon steel.
Under such circumstances, there is a strong demand for the development of an Fe-Ni alloy sheet for a shadow mask, which is excellent in etching pierceability and permits certain prevention of seizure during the annealing of the Fe-Ni alloy sheet, and a method for manufacturing same, but such an alloy sheet and a method for manufacturing same have not as yet been proposed.